1. Field of the Invention
This invention relates to a method of making a polylactic acid based article, more particularly to a method of making a heat-resistant polylactic acid based article using a thermoforming process.
2. Description of the Related Art
The research in biodegradable material is under progressive development in order to solve the problem of plastic waste that causes serious environmental impacts. Polylactic acid (PLA) is one of the most valuable biodegradable materials because raw materials of PLA are completely plant materials. However, PLA has a poor heat-resistant property which limits the applications thereof.
Generally, PLA is subjected to a long-term heating treatment under an appropriate temperature so as to increase the crystallinity and improve the heat-resistant property thereof. Because the crystallization rate of PLA is relatively slow which leads to a longer cycle time for formation, PLA needs a relatively longer time to increase the crystallinity thereof. However, during a heating treatment at an elevated temperature, the cycle time of PLA would be increased, thereby resulting in an even longer time for increasing the crystallinity of PLA and a higher manufacturing cost that can not meet commercialization demands.
In order to solve the aforesaid problems, crystal nucleating agents such as talc powder, isinglass, and low molecular weight plasticizer are mixed with PLA during manufacture so as to accelerate crystallization rate of PLA. However, the cycle time is still too long.
In addition, in order to achieve desired heat resistance and impact resistance and higher productivity, U.S. Patent Application Publication No. 2005/0165142 discloses a PLA composition containing PLA, a polyester, and a crystal nucleating agent. The optical purity of the polylactic acid and a residual lactide amount in the polylactic acid should be precisely controlled. Moreover, as shown in Table 2, the temperature inside a die (i.e., heat temperature) is 120°C. or more. The relatively high die temperature will cause poor yield rate due to sticking phenomenon that occurred between the die and a final article or uneven wall thickness of the final article. Besides, when the temperature inside the die is reduced to, e.g. 120° C., the time for heat treatment has to be increased to 15 seconds (see Example 9 of Table 2). In Comparative Example 12 of Table 2, when the temperature is reduced to 100° C., the time for heat treatment has to be increased to 60 seconds, and the final article has poor heat resistance. Therefore, the commercial desire for low die temperature and short time for heat treatment can not be accomplished in this publication.
U.S. Patent Application Publication No. 2009/0311511 discloses a PLA composition and a process for making a thermoformed PLA device. The PLA composition includes 100 parts by weight of PLA, 0.1 to 3 parts by weight of an organic crystal nucleating agent, and 0.1 to 7 parts by weight of a crystallization accelerator. The thermoforming process includes preheating a PLA based sheet containing the PLA composition to a temperature of 60° C. to 130° C. so as to increase the crystallinity of the PLA based sheet to a range from 20% to 50%, followed by a thermoforming step. The preheating step allows a shorter duration of the successive thermoforming step, which may overcome the aforesaid drawback of the relatively long cycle time. However, an excessively increased crystallinity may adversely influence extensibility of the preheated PLA based sheet, thereby resulting in a lower yield of the thermoforming process and deformations such as cracks on the thermoformed PLA device.
US 2009/0186990 discloses a polylactic acid based heat-resistant sheet suitable for molding. The polylactic acid based heat-resistant sheet contains a polylactic acid and a polylactic acid-based copolymer, and an optional amide crystal nucleating agent. In this publication, a molded article prepared from the polylactic acid based heat-resistant sheet has a crystallization index of 20 J/g˜35 J/g. From the disclosure of the publication, the object of the publication is to reduce the time for thermoforming by increasing the degree of crystallization of the polylactic acid based heat-resistant sheet. Therefore the problem of lower yield and deformations still remains.
The inventors of this invention find that, in a method for producing an article from the PLA composition containing PLA, polyester, and a crystal nucleating agent, the temperature and crystallinity are important factors for controlling properties of the article. In other words, when the temperature and crystallinity are not within a specific range, the article thus made would have inferior properties, e.g., poor thermal resistance, poor dimensional stability, etc.